1. Field of the Invention
The present invention relates to a color image forming apparatus provided with a multi-laser beam.
2. Description of the Related Art
There is known an electrographic process as an image printing process used in a color image forming apparatus such as a color printer or a color copy machine. The electrographic process is configured to forma latent image on a photosensitive drum using laser beams and develop the latent image with a charged color material (hereinafter, called toner). The printing of the image is performed by transferring the image developed by the toner on a transfer paper for fixation.
Recently there has been increasing an image forming apparatus equipped with a multi-laser beam for high-speeding of image formation in the color image forming apparatus of the electrographic process. Particularly there has been increasing a color image forming apparatus of a tandem process equipped with developers and photosensitive drums each number of which is the same as the color number of the toner to sequentially transfer images of different colors on an image transporting belt or on a print medium. There is known in public that in the color image forming apparatus of the tandem process using the multi-laser beam, there exist plural factors causing a misregistration, and therefore various types of measures are proposed for coping with each factor.
The factor is composed of ununiformity or a mount position shift of lens in a deflection and scanning device or a mount position shift of the deflection scanning device to the color image forming apparatus body. This position shift causes tilt or curvature of scan lines, and the degree or the direction of the curvature differs in each color to cause the misregistration. Hereinafter, in the present specification, information in regard to the scan line, such as polarity or a scan interval of a beam, which will be described later, in addition to the tilt of the san line and the degree and direction in the curvature thereof, is called a profile.
The tilt or the degree in the curvature of the scan line differs in each image forming apparatus, that is, each print engine, further each color. An example thereof is shown in FIG. 17A to FIG. 17D. In FIGS. 17A to 170, a lateral axis shows a position of the image forming apparatus in the main scan direction. A line 2000 linearly expressed in the main scan direction shows an ideal profile characteristic with no curvature. In addition, each of line 2001, line 2002, line 2003 and line 2009 shown by curvature lines shows a profile characteristic for each color. That is, line 2001 shows a characteristic of cyan (hereinafter, called C), line 2002 shows a characteristic of magenta (hereinafter, called M), line 2003 shows yellow (hereinafter, called Y) and line 2004 shows black (hereinafter, called K). A longitudinal axis shows a shift amount in the vertical scan direction (sub scan direction) to the ideal profile characteristic. As apparent from the figure, a curvature direction or a changing point of the curvature line differs in each color, and this difference appears as a misregistration in the fixed image data.
Japanese Patent Laid-Open No. 2004-170755 shows, as a measure method against the misregistration, a method where an optical sensor is used to measure a tilt and a magnitude of the curvature of each of the scan lines, a bit map image data is corrected to cancel out them and the corrected image is formed. According to this method, since the image data is electrically corrected, a mechanical adjustment member or an adjustment process at assembling is unnecessary. Therefore, it is possible to downsize the color image forming apparatus. The electrical correction of the misregistration is classified into correction in a one-pixel unit and correction in a unit of less than one pixel. FIG. 18 shows diagrams explaining the correction in a one-pixel unit. A longitudinal axis in FIG. 18 shows a sub scan direction of a sheet and a lateral axis therein shows a main scan direction thereof. (a) of FIG. 18 is a profile characteristic of a laser irradiation showing that the scan line is bent in the reverse direction of the sub scan direction to the main scan direction. The correction in the one-pixel unit is made such that, as shown in (b) of FIG. 18, a pixel is offset in the one-pixel unit in the sub scan direction corresponding to each correction amount of the tilt and the curvature. It should be noted that, in the following description, a position for offsetting is called a scan line changing point. That is, in (a) of FIGS. 18, P1 to P5 correspond to scan line changing points. (c) of FIG. 18 is a diagram showing an image scanned after correction which is offset in the one-pixel unit in the sub scan direction.
The correction in a unit of less than one pixel is, as shown in FIG. 19, made by adjusting a gradation value of the bit map image data with pixels in the sub scan direction after and before the gradation correction. That is, as in (a) of FIG. 19, in a case where the profile characteristic is bent in an upward direction, the bit map image data before the gradation correction is treated in a direction opposite to the upward direction, that is, in a downward direction at the sub scan side. Making the correction in the unit of less than one pixel according to such method enables an unnatural step in the boundary of the scan line changing point caused by the correction in the one-pixel unit to be eliminated, thus achieving smoothing of the image.
In a case of applying the aforementioned conventional technology to the image forming apparatus equipped with the multi-laser beam, the electrical misregistration correction has to be made to various types of profile characteristics of the respective beams constituting the multi-laser beam.
On the other hand, since a circuit scale used for the correction increases, a simplified correction unit is required to be mounted in a low end product. For example, in consideration of elimination of the number in the scan line changing points and tilts or an average value or the worst value of the curvatures in plural laser beams, simplification of the correction unit, such as implementation of a single correction, is performed. In this case, since an optimal correction corresponding to the tilt or the curvature characteristic of each beam is not made, there occurs a problem that the image is degraded.
FIG. 20 is a diagram explaining a mechanism in which the image degradation occurs in a case of using a multi-laser beam formed of four beams. (a) of FIG. 20 shows a profile characteristic of each of the four beams. In this example, each of the first and second beams from the top has the curvature in a direction opposite to the sub scan direction and each of the third and fourth beams from the top has the curvature in the sub scan direction. (b) of FIG. 20 shows a trace of each beam in a case of printing using the four beams. (c) of FIG. 20 and (d) of FIG. 20 are enlarged diagrams of sections shown in (1) and (2) in (b) of FIG. 20. This diagram shows that, in a case where the optimization is not carried out based upon the profile characteristic of each beam, there appear in the end sections, one portion where dots overlap so that a print density is high and the other portion where a location where a dot is supposed to be struck is blank so that the print density is low. Of course, in some cases, all the four beams have curvatures in a same direction. Meanwhile, there are some cases where the fixing speed of the toner is changed, such as a case of designating a sheet other than a plain paper (for example, thick paper) at printing or a case of selecting a gloss-up mode for increasing gloss.